Wind mills are assisting mankind to convert wind energy into electrical energy. Modern wind turbines are capable to convert wind energy into electrical energy under various wind conditions. This is due to the blades which are developed using state of the art aerodynamic analysis and other performance enhancement equipment.
The blade construction and design is one of the major factors deciding the wind force required to rotate the blades. The wind blades have airfoil cross sections consisting of different sizes and shapes from root to tip. A force is produced when the fluid moves over the air foil. The component of this force perpendicular to the direction of motion is called lift, and the component parallel to the direction of motion is called drag. Usually the wind turbine blade is kept in a tilted manner, with a continuous twist from root to tip in order to efficiently align with the relative wind speed.
An airfoil has a leading edge and a trailing edge. The upper surface extends from the leading edge to the trailing edge along the top surface whereas the lower surface extends from the leading edge to the trailing edge along the bottom surface. The straight line extending from the leading edge to the trailing edge is referred to as the chord. The distance between the upper surface and the lower surface perpendicular to the chord is the airfoil thickness, which varies along the chord. The line defined by the midpoint of the thickness is the mean camber line. Conventionally, the dimensions of an airfoil are often defined with reference to the chord length. For example, the maximum thickness of an airfoil is often defined as a percentage of the chord length, the location of the maximum thickness and the maximum camber is typically defined as a percentage of the chord length (measured from the leading edge), and the maximum distance between the chord and the mean camber line, which is a measure of the curvature of the airfoil, is referred to simply as the “camber” or the “maximum camber” and is typically defined as a percentage of the chord length.
With regard to wind turbine blades, the poor lift characteristics of conventional airfoils at low Reynolds numbers delay the starting and reduce the efficiency of wind turbines working under low wind speed conditions. In order to start generation, the rotor of the wind turbine should develop sufficient aerodynamic torque to overcome the resistive torque of the generator. This aerodynamic torque has to be derived from the lift force developed by the blades. However, under low Reynolds number conditions, lift characteristics of normal airfoils are degraded due to the formation of laminar separation bubbles.
Such a separation bubble is caused by a strong adverse pressure gradient (pressure rise along the surface), which makes the laminar boundary layer to separate from the curved airfoil surface. The pressure rise is related to the velocity drop towards the trailing edge of the airfoil, which can be seen in the velocity distribution of the airfoil through Bernoulli's equation.
The boundary layer leaves the surface approximately in tangential direction, resulting in a wedge shaped separation area. The separated, but still laminar flow is highly sensitive to disturbances, which finally cause it to transition to the turbulent state. The transition region (not exactly a transition point) is located away from the airfoil at the outer boundary of the separated flow area. The thickness of the now turbulent boundary layer grows rapidly, forming a turbulent wedge, which may reach the airfoil surface again. The region where the turbulent flow touches the surface again is called reattachment point. The volume enclosed by the regions of separated laminar flow and turbulent flow is called a laminar separation bubble. Inside the bubble the flow may be circulating, the direction near the airfoil surface may even be the opposite of the direction of the outer flow. There is almost no energy exchange with the outer flow, which makes the laminar separation bubble quite stable. The separation bubble thickens the boundary layer and thus increases the drag of the airfoil. The drag increment can be several times the drag of the airfoil without a separation bubble. Lift and Moment are also influenced by a laminar separation bubble.
One way to avoid or minimize the adverse effect of the laminar separation bubble is to promote earlier transition of the flow from laminar to turbulent. Some methods to achieve this transition are providing tabulators or trips over the surface of the airfoil. A mechanical turbulator consists of a modification of the airfoil shape, which causes large local gradients in the shear stress of the fluid, which finally cause transition. It can be attached to the surface as a straight tape strip (also called a 2D turbulator) or it can be distributed in a certain area like zig-zag tapes or single bumps, spaced equally. A different possibility is a wire, which is mounted on small struts in front of the leading edge. This device is less sensitive to changes in angle of attack, but causes larger additional drag. Typical values for turbulator height on model aircraft range from 0.2 for higher Reynolds numbers to more than 1mm for free flight models. However, these extra fittings over the airfoil surface may create undesirable disturbances to the flow.
The two major parameters with which the design process of wind turbine blades should start are the design wind speed at which the turbine is expected to work at its highest efficiency point and the tip speed ratio (ratio of the velocity of the rotor tip to the wind velocity) at the design point. These two factors are very important in deciding the size and shape (chord and twist along the blade length). When a wind turbine is designed specifically for a location, it could be possible to choose the design wind speed and tip speed ratio in such a way that, the operating conditions of the turbine match well with the prevailing wind regime at the candidate site to drive highest overall system efficiency. These factors can be determined by analyzing the historic wind profile at the sites, both in terms of the strength and the distribution of the prevailing wind.
Therefore, there is need of a blade having a unique design which can minimize the effect of the laminar separation bubble without the use of any extra fittings or turbulators which can increase the efficiency of the blade and decrease the cost of overall wind turbine. There is also requirement of a design method which can be utilized to build a blade with respect to the location or the place where in the wind turbine is proposed to be installed so that the overall performance of the system at candidate sites can be maximized.